1. Field of the Invention
The present invention relates to a magnetic recording medium with improved running characteristic.
2. Related Background Art
A magnetic recording medium having a magnetic recording layer composed of a ferromagnetic metal film, a ferromagnetic oxide film or a ferromagnetic nitride film shows excellent high density recording capability. Such film can be composed, for example, of Co, Co-Ni, Co-Cr, cobalt oxide or barium ferrite. Such films can be produced by thin film deposition processes, such as vacuum evaporation, sputtering or ion plating which is featured by good production efficiency and good control on electromagnetic characteristics.
The magnetic recording media prepared with such thin film deposition processes show excellent electromagnetic properties, but are not yet practically acceptable since they are significantly inferior in the running durability to the conventional coated media. Particularly the medium utilizing Co-Cr alloy film has yet to be improved in the practical durability.
Attempts for improving the running durability of such media utilizing metal magnetic film has been made in following two directions.
The first approach is to effect an improvement on the surface of the magnetic recording layer, either by providing a protective function or a lubricating function thereto, by modifying materials or process.
The second approach is to form minute coarseness on the surface of the magnetic recording layer, or on the surface of substrate not having the magnetic recording layer. Very small projections or undulations formed on the surface of the magnetic recording layer significantly improve the durability in comparison with the smooth surface.
However these methods inevitably result in an increased spacing loss, so that a highly advanced technology is required for improving the durability while maintaining the excellent electromagnetic conversion characteristics of the metal magnetic film.
The effort of the present inventors has been directed to the above-mentioned second approach.
A magnetic film prepared for example by sputtering on a substrate film having a coarse surface also shows a coarse surface reflecting the surface coarseness of said substrate film. It is already known that a film with such coarse surface contributes to an improvement in the running durability of the magnetic recording medium, as disclosed in the Japanese Laid-open Patent No. 127523/1985.
A coarse surface of an organic film can be obtained, for example, by mixing particles for forming projections into the raw material in liquid state before film formation, by coating dilute solution containing said particles after film formation thereby forming projections after drying, by utilizing precipitates from catalyst residue or soluble catalyst, or by forming very small reticulations through crystallization in the film formation. The last two methods are principally utilized for improving the slidability of polyester films and utilize process features specific to polyester films. The first two methods are suitable for heat-resistant films such as polyimide or polyamide which are formed by casting, but the method of forming projections by causing the filler particles to project is advantageous in the simplicity of process and in achieving a low-cost industrial production. However simple mixing of filler particles into an organic film, particularly a polyimide film, does not achieve improvement in the running performance and durability of magnetic recording medium, and a further improvement has been longed for.